There is a need to accurately position agricultural implements used for planting crops. This maximizes yield by avoiding overlapping and gaps between rows which result in lower yield and increased costs and crop damage. As farming becomes increasingly mechanized and farm implements larger and larger so too does the difficulty of making each pass match precisely against the previous one. Cultivators and harvesters typically have different number of rows from that used on the planter. This makes it critical to ensure spacing remains the same from one pass to the next, as when the next vehicle comes down the row it may lie between two passes of the previous vehicle.
The standard approach, borrowed from the days of horse drawn ploughs, has been to use mechanical means to make a furrow in the ground to indicate where to guide on the following pass. This has evolved into the use of hydraulically activate disk marker arms which automatically swing in and out at the end of the towed implement at the end of every pass through the field to make this guide furrow for the next pass. As tractors have become larger with increased towing power, as fields have become larger to increase efficiency, so too has the size of the towed implement. As the implement size increases so too must the size and complexity of this swing arm.
Several agricultural implement manufacturers have devised additional wand sensors on the towed implement to automatically feel the edges of this guide furrow and adjust the lateral position of the towed implement by way of a free swinging three point hitch. This mechanism allows for correction of positional errors of the tractor up to around +−6 inches.
For some years Differential Global Positioning Systems (DGPS) have been used as a tool for agricultural guidance. Improvements in DGPS accuracy have now attained accuracies below the one-inch level. In some systems, this allows the driver to steer the tractor using a guidance display with improved precision. Several auto-steer systems are currently in use which take this guidance information and apply it directly to the steering mechanism of the tractor. This alleviates the driver from the fatigue of continuously responding to visual signals to keep the tractor on track, with the towed implement following behind.
This, however, does not control the towed implement, typically containing the planting or ploughing mechanisms. As these towed implements have become larger so too have the lateral positioning effects, which can shift their position relative to the towed tractor. Even if the tractor remains faithful to it's steering commands, visual or auto-steered, this can result in the tractor driving straight but the implement not remaining on track. Unfortunately, with the implement out of track, degraded performance results. On sloping ground, or ground with variation in texture or moisture content, this can give serious errors in the positioning of the furrows or plant positioning.